This is a method for generating a multi-centered halftone pattern, having a number of dots in a repeating pattern, in combination with a "write white" xerographic process to minimize visible artifacts in the printed output.
In the standard laser printer which uses the xerographic process to produce a printed page, the photoreceptor is charged up to a uniform voltage, and then a laser is used to discharge the photoreceptor to form the image. It is possible to have the laser discharge the areas that are to be black, "write black", or to discharge the areas that are to be white, "write white". Write black printers are preferred in the digital printing industry since this type results in a cleaner output hard copy. That is because, to the extent that the laser misses a spot because of dust or dirt, the artifact will appear on the output as a white spot. Since the printed material is most often black text on white paper, a white artifact will be less obvious. Also, as will be explained below, isolated black pixels in a write black system are larger than they should be, thus making a black line one pixel wide more visible than it would otherwise be. In a write black system, toner which is charged to a voltage similar to that of the charged photoreceptor is then applied to the photoreceptor. Because of the potential differences, the toner will adhere where the receptor is discharged and not adhere where there is a voltage. The resultant toner image on the receptor is then transferred to a piece of paper to create the output hard copy.
This xerographic process reliably produces dots that are either completely black or completely white, but usually can not accurately produce gray levels. Therefore, to create the appearance of gray levels, a halftone pattern is commonly used in xerographic printers to show gray scale pictures. A halftone matrix is usually a square composed of a number of pixels. As a numerical example, let us assume a square of four pixels by four pixels, eight of which, varying from light to dark gray, are shown in FIG. 1. Now, to create the appearance of white, all pixels are white. To create the appearance of lighter shades of gray, one, two three or four of the internal pixels can be black and the outer pixels can be white. The result is a black dot in the middle of the square. Subsequently, continuing to make the outer pixels black will continue to make the square darker, until the point is reached where all of the pixels are black. In appearance, the process is that of having a white square with a central black dot. By making the dot either smaller or larger, the color of the overall appearance can be made to look lighter or darker.
A problem with this arrangement is that there are only 17 levels of gray that can be reproduced, the number being limited by the number of pixels that can be turned on in a four by four pixel square. In the halftone rendition of a gray scale image, there will be a visible contour artifact at those points in the picture where there is a change from one gray level to another.
This problem can be reduced by going to a larger square. For example, an eight by eight pixel square will have 65 levels of gray. See FIG. 2 which shows two such squares, each having six black pixels. With 65 levels, the changes in darkness from one level to another will have become less visible to the eye. However, with such a large square, the size of the dot and the space between dots also become bigger, so that the resultant image coarseness becomes more visible.
One solution to this problem is to use a multicentered dot as in FIG. 3. In both FIGS. 2 and 3, each eight by eight pixel dot has six ON pixels, but in the FIG. 3 version there will be four dots within each square instead of one. This arrangement doubles the number of dots per inch in each direction, and the dot size and the distance between dots is smaller, so the result has fewer visible contours and looks less grainy, while still maintaining a large number of gray levels. This is therefore an improvement over both systems described above.
However, a close inspection of the output picture will still show a slight pattern of graininess or visible texture at light shades of gray. Here, the eye is detecting the pattern of one dot being slightly larger and the next being slightly smaller. One method that has been tried to obscure this artifact is to assign pixels to the dots in a slightly random pattern instead of in the regular pattern as shown here. This method is successful in breaking up, and therefore obscuring, the pattern, but does not preserve the uniform density that a flat color should have, the result being a picture that has a splotchy appearance.
An additional characteristic of the eye is that it is more sensitive to these artifacts at the lighter shades of gray. Therefore, what is needed is a system which keeps the regular multicentered halftone pattern, but which reduces its obviousness, especially at the lighter shades of gray. In addition, the system should also be adaptable to four color printing.